1. Field of the Invention
The present invention relates to a method for correcting the welding path of a welding torch in automatic welding.
2. Description of the Related Art
As shown in FIG. 1, an automatic welding apparatus which automatically welds two base metals includes a welding torch 10 supported by a torch holder 2 coupled to an arm 1 of a welding robot (not shown), and a controller 3 for controlling the welding robot. In a welding process, the welding torch 10 moves integrally with the arm 1 of the welding robot which, operates based on signals from the controller 3. A high voltage is applied between the welding torch 10 and base metals 20 and 30 being welded, in order to generate an arc therebetween. A welding wire 5 is provided from a wire reel 4 to the welding torch 10, and projects appropriately from the welding end of the welding torch 10. The end of the wire 5 projecting from the welding end of the welding torch 10 is melted by the arc current, and then hardens on welding surfaces 21 and 31 of the respective base metals 20 and 30, to combine the base metals 20 and 30.
During the above welding process, arc sensing is performed to confirm whether the welding torch 10 moves precisely tracing a welding center line, i.e., a boundary line between the two base metals 20 and 30, based on data of arc current values. The data is obtained by weaving the welding torch 10, that is, moving the welding torch 10 from side to side as it proceeds along the weld, and monitoring the arc current using a current monitor 6 according to the change of distance between the welding torch 10 and the welding surfaces 21 and 31.
The principle of the arc sensing is detailed with reference to the drawings. As shown in FIGS. 2 and 3, when the first base metal 20 and the second base metal 30 contacting each other are automatically welded, the welding torch 10 proceeds along the welding center line in a previously taught direction as indicated by an arrow "A" and concurrently performs weaving the same distance to the left and right as shown in dashed lines, in order to weld the two base metals 20 and 30. In the middle of the weaving, the welding torch 10 reciprocates between the farthest points from the center of weaving "C" toward and above the respective first and second base metals, i.e., a first position and a second position. In the case that the welding torch 10 precisely traces the welding center line 25, the center of weaving "C" is located perpendicularly above the welding center line 25. Also in this case, a vertical distance "H1" between the welding end of the welding torch 10 and the first base metal 20 at the first position is the same as a vertical distance "H1'" between the welding end of the welding torch 10 and the second base metal 30 at the second position. A vertical distance "H" is the maximum distance between the welding end of the welding torch 10 and the welding center line 25, at the position where the welding torch 10 is coincident with the welding center line 25.
FIG. 4 is a graph showing the change of vertical distance between the welding end of the welding torch 10 and the welding surfaces 21 and 31 of the first and second base metals 20 and 30, as the welding torch 10 weaves from the first position to the second position. Here, the horizontal and vertical axes "t" and "d" represent time and vertical distance, respectively. Referring to FIG. 4, it is indicated that the welding torch 10 disposed at the first position is moved to the second position over a predetermined time "t1" while performing weaving. Here, the vertical distance between the welding end of the welding torch 10 and the welding surface 21 of the first base metal 20 increases until the welding torch 10 reaches the center of weaving "C", i.e., time "t1/2". While the welding torch 10 moves to the second position after having passed the center of weaving "C", the vertical distance between the welding end of the welding torch 10 and the welding surface 31 of the second base metal 30 decreases.
Meanwhile, the arc current decreases as the distance between the welding end of the welding torch 10 and the welding surfaces 21 and 31 of the base metals 20 and 30 increases. Accordingly, the arc current varies as shown in FIG. 5 while the welding torch 10 moves from the first position to the second position. The current monitor 6 monitors the arc current varying as above and transfers information on the variation to the controller 3. The controller 3 calculates the integral of the current during the movement from the first position to the center of weaving "C", and the integral of the current during the movement from the center of weaving "C" to the second position, and compares these two integral values. When the two integral values are the same, the center of weaving "C" of the welding torch 10 is determined to coincide with the welding center line 25, that is, the welding torch 10 exactly traces the welding center line 25.
However, as shown in FIG. 6, when the weaving center "C" of the welding torch 10 is misaligned with the welding center line 25, a vertical distance H2, between the welding end of the welding torch 10 and the first base metal 20 at the first position, and a vertical distance H4, between the welding end of the welding torch 10 and the second base metal 30 at the second position, are different from each other. When the welding torch 10 is located at the center of weaving "C", the welding torch 10 is located above the second base metal 30. Here, the vertical distance between the welding end of the welding torch 10 and the second base metal 30 is indicated by "H3". During the movement of the welding torch 10 between the first position and the second position, the vertical distance between the welding end of the welding torch 10 and the welding surfaces 21 and 31 of the first and second base metals 20 and 30 varies as shown in FIG. 7. Accordingly, the arc current varies as shown in FIG. 8. Thus, the integral of the current between the first position and the center of weaving "C", and that of the current between the center of weaving "C" and the second position, are different from each other. Thus, the controller 3 determines that the welding torch 10 does not precisely trace the welding center line 25. When the difference between the integral values exceeds a predetermined value, signals are generated from the controller 3. Then, the robot arm 1 moves according to the signals, in a direction that can minimize the difference between the integral values, so that the position of the center of weaving "C" is corrected to be positioned perpendicularly above the welding center line 25.
As shown in FIG. 9, when welding is performed in a state in which a proceeding direction "A" taught to the welding torch and the welding center line 25 are not coincident, the position of the weaving center is continuously corrected as explained above. That is, when the welding torch moves such that the center of weaving thereof follows the direction indicated by the arrow "A" and reaches a position "A1", at which the distance between the welding torch and the welding center line 25 becomes greater than a predetermined value, the above-mentioned correction of the weaving center is performed such that the center of weaving is located at a first correction position "B1" on the welding center line 25. Thereafter, the welding torch further proceeds following the direction "A", and corrections of the weaving center are made again at positions "A2" and "A3", at which the distance from the welding center line 25 becomes greater than the predetermined value, to second and third correction positions "B2" and "B3". Such corrections of the weaving center, i.e., correction of a welding path, are continuously performed during welding.
However, according to the conventional method for correcting a welding path, the weaving of the welding torch is always in a direction "W1" perpendicular to the direction "A" in which the welding torch proceeds. When the welding torch weaves in the direction "W1" and when the welding center line 25 is skewed with respect to the direction "A" of the welding torch as shown in FIG. 9, the distance of weaving perpendicular to the welding center line 25, i.e, the distance between the first position and the second position, decreases. Thus, the integral of the current during the movement between the first position and the weaving center, and the integral of the current during the movement between the weaving center and the second position, are reduced, and thus the difference between the two integral values decreases. Therefore, since the correction of the welding path of the welding torch is not performed at the instant the correction should be performed, the precision of the welding process is lowered.